The Collaborative International Dictionary of English v.0.48:

Water spider \Wa"ter spi"der\ (Zool.)
   (a) An aquatic European spider (Argyoneta aquatica) which
       constructs its web beneath the surface of the water on
       water plants. It lives in a bell-shaped structure of
       silk, open beneath like a diving bell, and filled with
       air which the spider carries down in the form of small
       bubbles attached one at a time to the spinnerets and hind
       feet. Called also diving spider.
   (b) A water mite.
   (c) Any spider that habitually lives on or about the water,
       especially the large American species (Dolomedes
       lanceolatus) which runs rapidly on the surface of water;
       -- called also raft spider.
       [1913 Webster]